Micromagnetic Simulation Of Nanowire (Nanoring)

Nanowires and nanorings have significant advantage in advanced storage media, spintronic and advanced microwave technologies. Thus, the magnetic properties of nanowires and nanorings are studied in this paper. The static magnetic structure of a single defective Ni80Fe20nanowire has been investigated by magnetic force microscopy (MFM), micromagnetic simulation and MFM image simulated computation. The magnetic properties of nanoring (arrays) have been studied by using micromagnetic simulations. The primary works of the dissertation are described as follows:The static magnetic structure of a single defective Ni8oFe?o nanowire has been investigated by magnetic force microscopy. The result shows that the defects of the shape have an effect on magnetic state of the nanowire. In order to check the influence of defects of the shape on the magnetic state, micromagnetic simulation and MFM image simulated computation are introduced. Two model systems are studied:(Ⅰ) model I:idealized nanowire structure and (Ⅱ) model II:with the defects of the shape at the ends of the nanowire. The simulated computation result of model Ⅱ is good agreement with experimental result. Due to defects of the shape of the nanowire, magnetization distributions of the nanowire have been changed. It may have important guiding significance in the sample preparation and application. According to above work, there are two boundary domain walls at the ends of nanowire. Boundary domain walls which free stray field will affect the adjacent units’ magnetic properties. On other hand, the defects of the shape play an important role on magnetic configurations of magnets. Therefore, we study the effect of size dimensions and the defects of the shape on the magnetic properties in nanoring as follows.Firstly, two systems of Co and Ni80Fe20nanorings are studied by micromagnetic simulations:1) Rings with200nm diameter and30nm thickness, with different width (30nm-100nm). The emphasis is placed on the effect of ring width on the magnetization reversal and the stability of the vortex. The simulated results indicate the wider rings enter vortex state earlier. As the width becomes wider, the ring displays a smaller vortex window i.e., the stability of the vortex become weaker. The double switching process is found in wider rings:onion-vortex-reverse onion.2) Rings with200nm diameter and10nm width, with different thickness (10nm,20nm,50nm) and rings with200nm diameter and30nm width, with different thickness (10nm-50nm). The emphasis is placed on the effect of ring thickness on the magnetization reversal and the stability of the vortex. As the result shown, the smaller and thicker rings show a single reversal mechanism: onion-reverse onion. For the wider and thicker rings, there is double reversal mechanism. As the ring becomes thicker, it displays a wider vortex window and smaller switching field. Compared with Ni80Fe20nanoring, there are more local vortices of Co nanoring in magnetized process.The interaction effect of5×5Co nanoring arrays (diameter of250nm, width of30nm, thickness of30nm, and pitch of200nm,400nm,600nm,1000nm) have been studied. As same as the single ring, nanoring arrays show double reversal mechanism. But the switching field of nanoring arrays is larger than single ring. The stability of the vortex in arrays is weaker. When the pitch is small, due to the stray-field-free onion state to vortex state, some of the rings start to switch to the vortex state while others remain at the onion state. The arrays with larger pitch has larger vortex window and is closer to the single nanoring’s switching due to less interaction.Lastly, effects of the defects of the shape and the magnetic field direction on the magnetic properties in nanoring are studied.1) We design three systems of nanorings which have the defects of shape:with the inner hole as an ellipse, introduced a notch in the ring as a domain wall pinning, shifting the inner ring center from the middle of the ring. The results shown, two mechanisms have been proposed to control the vortex chirality in nanorings, by pinning one domain wall or by control the movement direction of the two domain walls.2) We also study the controllable vortex chirality of asymmetric Co nanoring (diameter of140nm, width of20nm and60nm, thickness of10nm) with the various magnetic field direction. When the field in the0°and70°, their have opposed vortex chirality. Thus, the simulated results indicate that the direction of magnetic field and the defects of shape will help control the chirality of vortex.3) The effects of the number and the location of notches on the forming of flux-closure states in bi-rings with applied field in the x direction and y direction had been investigated using micromagnetic simulation. For the bi-rings with a notch and the bi-rings with two notches about y axis symmetric, the order of forming flux-closure state in each ring can be controlled. But the flux-closure state forms simultaneously in each ring for the bi-rings with two notches about x axis symmetric. For the bi-rings with two notches asymmetric, only one ring can form flux-closure state in y direction field and no flux-closure state can be found in each ring for applied in x direction field.Thus, the defects of the shape and size dimensions play important roles on magnetic properties of nanoring. It may have important guiding significance in the sample preparation and application.